Teletypewriter cord circuit



C. A. DAHLBOM ET AL TELETYPEWRITER CORD CIRCUIT Oct. 10, 1944.

Filed NOV. 7, 1941 mm. QN N Patented Oct. 10, 1944 TELETYPEWRITER CORD'CIRCUIT Carl A. Dahlbom, Brooklyn, Fullerton S. Kinkead, New York, andGeorge A. Locke, Glenwood Landing, N. Y., assgnors to Bell TelephoneLaboratories, Incorporated, New York, N. Y., a corporation of New YorkApplication November 7, 1941-, Serial No. 418,142 9 claims. (C1. 1781-2)This invention relates to teletypewriter switching systems andparticularly to an improved manual teletypewriter cord circuit forextending connections between calling and called teletypewritercircuits. More particularly the invention is an improved cord for use ina straightforward teletypewriter switching system. One of the mostimportant aspects of a cord circuit suitable for use in astraightforward system is that in addition to transmitting telegraphcommunication signals, it must be capable of transmitting supervisorysignals received at one end of the cord through the cord to the otherend thereof automatically without the intervention of an operator. m

The disclosure of Patent 2,228,890 to F. J. Singer, January 14, 1941,includes cord circuits arranged for use in a straightforwardteletypewriter vswitching system. The cord circuit herein may beregarded as an improvement over the cord circuits shown in the Singerpatent.

An object of this invention is the improvement of teletypewriterswitching systems.

A more particular object of this invention is the improvement of Ithecord circuit employed in manual straightforward teletypewriter switchingsystems.

A feature of the improved cord circuit is an arrangement wherein asupervisory signal, such as a disconnect signal, is operated by thesupervisory signal means associated with the calling end of a cord andsimultaneously the disconnect signal is transmitted directly through thecord to control supervisory signalV apparatus in the circuits associatedwith the originating end of the circuit. 'I'his is done automaticallywithout the intervention of the operator. Theoperator administering thecord circuit at a tandem position may be instructed to disconnect thecord in response to the supervisory signal without waiting for thereception of a disconnect signal from the originating operator. This-isa time saver in switchboard operation.

Alternatively, the operator at a tandem switchboard, along the route ofa straightforward call, may be instructed to disregard the disconnectsignal associated with the calling end of the cord and await adisconnect signal from the originating operator. Such signal would beregistered' by the supervisory signal apparatus associated with theanswering end ofthe cord. `This alternative method of operation wouldmake it possible for the originating operator to recall the called partyif it were required. Y

A further feature of this invention is the inclusion in a cord circuit,arranged to provide straightforward supervision, of means fordiscriminating between various connected circuits to which supervisorysignals are and' are not required to be sent.

v pletely independent and which cord* simultaneously indicates andtransmits supervisory signals, of other features, such as interferingcord lock-out, flashing recall, flashing recall release, anddiscrimination between connected circuits, etc.

These and other features will become apparent from the'followingdescriptions, when read with reference to the associated drawing showingthe improved straightforward cord circuit of this invention, as well asfrom the appended claims.

General The invention herein is limited to 'an improvedtel'etyiie'writer cord circuit per se. The detailed description istherefore limited to a description of the detailed operation of the cordcircuit. In its relation to cooperating circuits, the operation ofthecord circuit herein is disclosed by describing'the conditions imposed bythe cooperating circuits on the various cord circuit conductors whichconnect to cooperating circuits, to control the cord in the performanceof its various functions.

Itis pointed out that, in general, when it is explained that the cordcircuit is connected to a particular circuit, to exemplify a particularoperation within the cord, the conditions encountered on the conductorsto which the various conductors are connected are described, but the.

cooperating conductors and apparatus, in the circuit external to thecord circuit of this invention, are not shown, as they are not part ofthe invention herein and their showing is not thought necessary to anunderstanding of the invention herein.

The cord circuit herein is arranged l. To interconnect a callingsubscribers line directly to a called subscribers line when both linesare terminated in the same teletypewriter switchboard.

2. To extend a call originating in a calling subscribers line, in theswitchboard where the cord l circuit herein is locatedthrough a trunkcircuit extending to a distant central teletypewriter switching stationfor completion or' further extension.

3. To extend a call incoming over a trunk circuit-from a distant centralteletypewriter switching station by means of another trunk circuit to'another distant central teletypewriter' switching station for completionor further extension.

4. To complete a call incoming over a trunk circuit from a distantcentral telety'pewriter switching station directly to a calledsubscribers' line having a termination in the switchboard where the cordcircuit herein is located.

In performing all of the above functions, the incoming call is answeredwith the answering plug l. The operators position circuit is connectedtemporarily to thevcord. The operator communicates with the callingsubscriber or'with the distant central station operator through theanswering end of the cord by means of the operators position circuit.The call is completed or extended by means of the calling plug Il. The

Ordinarily, in the case of tandem connections, involving the connectionof subscribers lines through two or more manual central switching oces,only the operator at the office where the Vcall originates will respondto the supervisory signals. Operators at intermediate and completingoices in the chain will be instructed to disregard such supervisorysignals and await a disoperators position circuit is disconnected fromthe cord to serve withother cords. The cord circuit remainsinterconnected in the established circuit until communication betweenthe calling and called parties is ended. Asupervisory signal, called adisconnect signal, is operated in thek cord when communication isnished. The operator then takes down the cord.

The cord circuit is arranged to provide a flashing recall signal at eachend of the cord so that a central station operator may be signalled bythe calling or called party or by a distant central station operator aslong as the cord circuit forms a link in the connection. K

When the cord circuit herein is serving as a link at an intermediatecentral station in a tandem connection, it is arranged to receive adisconnect signal from a circuit connected to calling plug l l, andimmediately pass the supervisory signal, through a channel in the cord,so that the supervisory signal is impressed through the answering plug Ion a trunk circuit extending toward the originating central station.Thus, in response to the reception of the supervisory signal, on thecalling end of the cord, the supervisory signal is impressed on a trunkcircuit connected to the answering end of the cord, to control asupervisory signal in the cord circuit at a distant oice where the calloriginated. It is to be understood that there may be a number, such asthree or four or more, central switching stations connected in tandem insuch a connection. The importance of passing the supervisory signaldirectly through the successive cords automatically should be readilyappreciated. It saves operating effort and it saves both operating timeand idle line time.

The cord circuit is arranged so that the supervisory channel and thetelegraph communication channel are maintained separate within the cordcircuit. This distinguishes it from other cords in which the telegraphrepeater relays in the cord operate in the transmission of disconnectand recall signals and discrimination between communication signals andsupervisory signals is obtained through slow release relays havingwindings in the main transmission path. These relays are held operatedwhile ordinary communication signals are being transmitted through themain transmission path. Disconnect and recall signals in such cords areof sufli-cient duration to permit the slow release relays to releaseA tobring in the supervisory signals. In such cords the supervisory signalsare subject to false operation which is avoided when the channels areseparate as in the cord herein.

As mentioned above. a distinguishing feature of the cord circuit is thatit is arranged so that while a supervisory signal, received through thecalling plug is being automatically transmitted through the cord to theanswering plug, for transmission over a trunk to another cord connectedthereto, a supervisory signal is simultaneously registered in thepresent cord itself.

connect signal from the originating operator, which will appear on thelamp associated with the answering end of the cord. Such operationpermits the originating operator to rering the called subscriber, ifdesired, while the chain connection through the tandem oices is stillestablished, By providing a supervisory lamp for the calling end of eachcord in the chain connection and arranging that the calling endsupervisory lamp in each cord is operated, in response to a supervisorysignal transmitted by the called party, while simultaneously thesupervisory signal is automatically transmitted through each of thetandem cords toward .the originating onice, it is possible to disconnectthe cords at each oilice as soon as the disconnect signal is registered,without waiting for a second disconnect signal to be automaticallytransmitted in the reverse direction as each cord, starting with theoriginating cord at the calling ofce is disconnected.

Which of the above two methods of operation is used with the cordcircuit herein is a matter of choice. The particular method used woulddepend upon the percentage of calls which were found to reqquire theattention of the originating operator to perform some service requiringthe complete connection through to the called subscriber at the end of acall and would vary for different areas and different types of service.

The cord circuit herein is arranged for direct ringing or throughringing. Direct ringing is performed by actuating a key in the cordwhich t connects a ringing supply circuit which furnishes 20-cyclealternating current to the calling end of the cord.V Direct ringing isused when the cord is used as the originating cord on a tandemconnection. It is also used when the cord is used to interconnect twosubscribers directly. This is possible when both the calling and calledline terminate at the same switchboard. l v By through ringing is meantringing by re,- mote control. Through ringing is used on tandemconnections. The ringing through the intermediate and completing cordsis performed by remote control by operating a key in the cord at thelcentral oilice to which the calling subscriber is directly connected.

The through ringing arrangement in the present cord operates as follows:Twenty-cycle alternating current is connected to the tip conductor ofthe. calling end of the originating cord manually by the operation of akey.- The ringing current is transmitted over the tip conductor of thecalling'end of the cord and thetip conductor of the trunk circuitinterconnecting the central offices and through the tip conductor of theanswering end of the intermediate cord or completing cord depending onthe number of central offices in the tandem connection. 'I'he 20- cycleringing current operates the cord circuit repeater relay associated withthe answering end of the cord. This connects 2.0-cycle reversals ofpositive and negative direct current through the winding of analternating current relay associated with the calling end of the samecord. This in turn connects another ZO-CycIe-alternating current'ringingsupply circuit to the calling end of the intermediate or completingcord. Thisis impressed on the called subscribers line in the case of acompleting cord. Or, alternatively, the procedure may be repeatedthrough as many tandem offices as required to a remote completingoffice. I

It will be observed that in this system of operation -cycle alternatingcurrent is transmitted over the trunks interconnecting central officesand over the called subscribers line to operate the ringer therein. Thismethod has been found more reliable particularly over long circuits than20-cycle reversals of positive and negative direct current used by cordcircuits presently employed.

Answering incoming call from subscriber line A line signal indicatingthat a call is incoming from a-subscribers line is answered by insertingthe answering plug I in the jack associated With the line, shown at theleft of the drawing. When the answering plug I is iirst connected to acalling subscribers line circuit, only two conductors connected to thecord plug I are engaged. These are the tip and sleeve conductors. Thetip and sleeve conductors are the top and bottom conductors of the plug.The ring or middle conductor is not engaged in responding to a call asthe corresponding conductor on the ring of a subscribers jack is openfor the calling condition. The sleeve conductor of the answering plug isconnected to ground through the winding of a relay in the sleeve circuitof the calling line and the circuit extends from the sleeve, or bottomconductor, of plug I through contact 8b, left-hand winding of relay 5and the winding of relay 4 to battery. Relays 4 and 5 in the cordcircuit operate. The operation of relay 4 establishes a circuit fromground through contact 4b and the winding of relay I3 to batteryoperating relay I3. The operation of relay I3 operates relay 2I frombattery through contact I3a, resistance 4B and the left-hand winding ofrelay 2| to ground. The operation of relay 2I closes ground to theartificial line circuits of repeater relays 22 and 23. The operation ofrelay 2l also provides holding battery for the line windings of relays22 and 23, when required, to hold the armatures of these relays on theirmarking contact. This will now be described in more detail.

Relays 22 and 23 are shown with their armatures in engagement with theirspacing contacts. Before plug I is inserted in a jack and before relay2I is operated, no current flows through any of the windings oi relays22 and 23. When plug I is inserted into a jack the tip of plug I isconnected to the tip of the associated subscribers line circuit. Whenthe subscribers line circuit is closed for the marking condition, a loopis established through the line circuit and the return conductor isterminated in negative battery. From the tip of plug I the circuitextends through contact I3d, resistance 39, contacts ISa and ISe inparallel, contact 20h and 20e in parallel and the bottom winding ofrelay 22 to the armature of relay 23. When the armature of relay 23 isin engagement with its spacing contact 23s, as shown, the circuitextends through resistance 53 to negative battery. Since both ends ofthe loop are terminated in battery of the same magnitude and polarityfor this condition, no current iiows in the loop and the bottom windingof relay 22 is deenergized. With relay 2| operated a circuit may betraced from ground through contact 2Ia, resistance 44, top winding ofrelay 22', armature of relay 23, contact 23s and resistance 53 tonegative battery. The effect of current flowing in this path energizesthe top winding of relay 22 and its effect actuates the armature ofrelay 22 to engage with its marking contact 22m. The operation of relay2I also provides a path from negative battery through resistance 4S,contact 2Id, resistance 42, contact I'Ic, contact ISa, resistance 43 andthe bottom winding of relay 23 to the armature of relay 22. When thearmature is in engagement with its marking Contact 22m, the circuit isextended through resistance 50 to positive battery. It will be observedthat the opposite ends of thispath are terminated in batteries ofopposite polarity so that the batteries are in series-aidingrelationship. Full line current, therefore, ows through the bottomwinding of relay 23. A circuit may also be traced from ground throughcontact 2 Ib, resistance 45 and the top windingof relay 23 to thearmature of relay 22. When the armature of relay 22 is in engagementwith its right-hand or marking contact 22m, the circuit extendsthrough'resistance 5G' to positive battery. The effect of currentflowing in the bottom or line winding cf relay 23 tends to actuate thearmature of relay 23 to engage with its left-hand or marking Contact.This effect is opposed by the eiiect of current flowing through the topwinding of relay 23 and tending to actuate the armature of relay 23 toengage with its right-hand or spacing contact 23s. The eiect of thecurrent in the bottom or line winding of relay 23 preponderates for thiscondition and the armature of relay 23 is actuated so as to engage withits left-hand or marking contact 23m. When the armature of relay 23engages with marking contact 23m, the subscribers loop is terminated inthe circuit extending through resistance 5I to positive battery. For themarking condition, therefore, when the subscribers loop circuit isclosed, since the line end is permanently terminated in negativebattery, and the cord end for marking is terminated in positive battery,the batteries are in seriesaiding relationship for the markingcondition. Full line current, therefore, ows through the bottom or linewinding of relay 22 for the marking condition. The effect of thiscurrent will maintain the armature of relay 22 in engagement with itsright-handV or marking'contact 22m. The direction of current flowthrough the top winding of relay 22 is reversed when the armature ofrelay 23 engages with its marking contact. The eiiect of current in thetop winding of relay 22 for this condition tends to actuate the armatureof relay 22 towards its left-hand or spacing contact 2 2s, but it isineffectual, since the effect of current in the bottom or line windingof relay 22 is dominant and the armature of relay 22 is maintained inengagement with its right-hand or marking contact 22m. When the armatureof relay 22 is in engagement with its right-hand or marking contact 22m,the direction of the flow of current through the top winding of relay 23is reversed. This tends to actuate the armature of relay 23 to engagewith its spacing contact 23s, but the effect of current in the bottom orline winding of relay 23s, tending to actuate the armature of relay 23towards contact 23m, is dominant and the armature of relay 23 ismaintained in engagement with its left-hand or marking contact 23m. I

In tracing the sleeve circuit in the cord, it was shown to extendthrough the left-hand winding of relay 5 and the Winding of relay 4 tonegative battery. This is a high resistance path. After relay has beenoperated a parallel low resistance locking path for relay 5 isestablished extending through the right-hand Winding of relay 5 andresistance 33 to negative battery. The object of this is to establish alow resistance sleeve circuit to provide protection against interferencecaused by the connection of more than one cord circuit simultaneously tothe same line circuit.

Prevention of interference when more than one cord is connectedsimultaneously to one line The manner in which interference is preventedwhen more than one circuit, such as, the cord herein, is connected tothe same line circuit simultaneously, will now be described.

If an interfering cord is plugged into one of the multiple jacks of theseized line circuit, the relay corresponding to relay 4 in theinterfering cod will operate, but the relay in the interfering cordcorresponding to relay 5, which is a marginal relay, will not operate,owing to the shunting effect of the loW resistance sleeve circuit of therst cord. Relay 4 of the interfering cord, when operated, operates relayI3 of the interfering cord in the same manner as described above. Theoperation of relay 4 in the interfering cord Ialso operates relay 3 inthe interfering cord, over a circuit corresponding to that which may betraced from ground through contact 4a, winding of relay 3 and contact 5bto battery. As explained above, the relay corresponding to relay 5 inthe interfering cord is not operated, and therefore the path throughcontact 5b is available for operating the relay corresponding to relay 3in the interfering cord.

The operation of relay I3 in the interfering cord in turn operates relay2I in the interfering cord. The corresponding path has heretofore beentraced. This energizes the biasing windings of the polar repeater relayscorresponding to relays 22 and 23 in the interfering cord. It.

also supplies negative battery to the line winding of the relaycorresponding to relay 23 in the .1

interfering cord. In the interfering cord the operation of relay 3establishes a circuit which may be traced from positive battery throughresistance 5I, contact 3a, resistance 39, contacts ISa and 19e inparallel, contacts D and 20e in parallel and the bottom or line windingof relay 22 to the armature of relay 23. For this condition the armatureof the polar relay in the interfering cord corresponding to relay 23will be actuated so as to engage with its marking contact. The

armature of the relay corresponding to relay 22 in the interfering cordwill be actuated so as to engage with its spacing contact. transmissionthrough the interfering cord.

In the operators position circuit associated With' the interfering corda signal is provided to indicate the interfering condition in thefollowing manner. In the interfering cord the operation of key 24 or key25 will operate relay I9 or relay 20 over obvious circuits. Thisconnects the operators position circuit at the home or adjacent positionto the interfering cord@ A circuit is then established yfrom negativebattery through resistance 32, contact 3b, and contact I9g or contact20g depending upon which key has been operated. The armatures associatedwith these contacts are each connected to a relay in its associatedoperators position circuit which controls a signal, such as thelightingof a lamp, to indicate the interfering condition. Upon observ-This prevents,

ing this the interfering'operator will disconnect the interfering cord.

During the interval while the interfering cord was connected, thepositive battery supplied from the Yinterfering cord through resistance5I and contact 3a to the tip transmission conductor .to operate thearmature of polar relay 22 in the interfering cord to spacing andprevent transmission through the interfering cord, was impressed alsothrough contact I3d of the interfering cord, relay I3 being operated, onthe tip of the plug of the interfering cord. The tip of the interferingcord is connected to the tip of one of the subscribers line jacks. Thesubscribers line jacks of a particular line are all connected inmultiple. The tip of the original cord is therefore connected inparallel with the tip of the interfering cord. As a result of this thearmature of polar relay 22 of the original cord is also actuated to itsspacing contact and maintained thereon as long as the interfering cordremains connected. Thus transmission through either cord is preventedwhile the interfering condition persists. When the interfering cord isremoved and the actuated typing key therein is restored to normal therelays in the interfering cord are all restored to normal. The armatureof polar relay 22 of the original cord is returned to its markingcontact land transmission through the original cord may be resumed.

Connection to operators position circuit A number` of the cord circuitsherein, such as fteen, may be located at a particular operators positionin a switchboard. The position is provided also with a single operatorsposition circuit. Adjoining positions in the switchboard are similarlyequipped with, say fifteen cords and one position circuit. When aparticular cord circuit is selected for use, it is necessary to connecta position circuit to that particular cord temporarily, so Vas toprovide means whereby the operator at a particular position maycommunicate with the subscriber land receive the number of the calledparty or Whatever direction is -to be passed to the operator. After theoperator has received this information and performed the necessaryswitching function or whatever function may be required, the positioncircuit is disconnected from the particular cord so as to be availablefor connection to other cords as required. The circuit herein isarranged in such a, manner that it may be connected to a positioncircuit at the particular position where the cord circuit is located, orit may be alternatively connected to an operators position circuit at anad- `ioining position. This feature is provided so that operators atadjoining positions may function as a team during periods of heavy load.

The invention herein does not include the operators position circuitwith which .the cord circuit herein cooperates. Each cord circuit at aparticular position is provided with two keys. One key connects theoperators position circuit located in .the same position at which thecords are located to the particular cord. The other key connects theoperators position circiuit in an ad- Joining position to the cordcircuit. If it is desiredV to connect the operators position circuitlocated at the same position as that at which the cord circuit islocated to the cord, key 24 is operated. This establishes a circuit fromground through contact 2da and the winding of relay I9 to batteryoperating relay I9. The operation of relay I9 opens the transmissionpath interconnecting the tip of plug I and the bottom winding of polarrelay 22 at two points, namely at contacts ISu andr contacts ISe and'connects the transmission circuit of the operators position circuit inseries between continuity contacts Illb and I9f. The operationof key'25establishes acircuitv from 'ground through contact 251 and the windingof. relay 2! to battery operating relay 20. This inserts thetransmissioncircuit of the operators position circuit at an adjoining positionkbetween the tip of plug I and the .bottom windingoi polar relay 22.Contacts 2Gb and Ze in series in the transmission path through theanswering end of the cord are opened and the transmission circuit'through the adjacent operators position circuit is inserted in seriesbetween continuity. contacts 29a and 261, The operation of relays IS and2i] perform other switching functions which will lbe describedhereinafter.

After the operator has communicated with the calling subscriber andobtained the necessary switching information she will perform theswitching function required and then restore'either key 24 or key. 25 tonormal releasing. relay I9 or relay 20. This disconnects the associatedposition circuit from the particular cord and reestablishes the originaltransmitting path between the tip ofplug l and the bottom winding ofre-lay 2l.

Busy test A busy test is made by Ltouching the tipl of either answeringplug I or calling plug II to a sleeve of a line jack before insertingthe plug into ther jack in order to guard against double connections.I-f the circuit associated with the line jackl is busy, due to theconnection of another cord to one of the multiple appearances ofthe jackat other positions in the board, negative battery will ybe `connected tothe sleeve of the jack from the sleeve of the associated answering orcalling plug of the other connected cord.

If an answering plug of a cord similar to the cord herein is inserted inoner of the multiple appearances of a jack, battery will be supplied tothe sleeve of that jack over a circuit which may be traced from negativebattery over parallel. paths. The rst branch extends through the windingof relay It; and the left-hand winding of relay 5. The secondV branchextends through contact 5a, Iresistance 33 and the right-hand winding oirelay 5. The branches join andthe path continues through contacts 8bandthe sleeve of plug l to-the sleeve ofV the jack. If the calling plug,such as II, of a cord circuit similar to the cord herein has alreadybeen inserted in one of the multiple appearances of the jack, negativebattery is supplied through4 the winding of relay Il andresistance B3 inparallel and the sleeve of plug Il to the sleeve of the jack. Attentionis called toV the fact that when the testing is performed by touchingthe tip of the plug to the sleeve of the jack the whole plug is notinserted into the jack and the relays controlled bythe sleeve circuitofthe cord which is testingv will not beoperated. Beforemaking: a busytest an operator will operate either key 24 or key 25 in the` particularcord to its alternate position;

It will be assumed first that the testing s-performed by touching thetip of plug I to the sleevey of a jack on which negative battery hasbeen impressed fro-in another cord. Negative battery on the sleeve of.'the jack isimpressed through" the tip of plug I, contact |31?l and, ifkey 244 has 'been actuated, through contact leg and over a. conductorwhich extends into the home operators position circuit, through thewinding of a relay connected thereto, to light a busy test lamp intheposition circuit as an indication of lthe busy condition. If key 25. hasbeenoperated so that relay 20, rather 'than relay I, has been operated,to connect the. cperators positionof. the adjacent position to theparticular cord, the circuit extends through contact 2tlg tocorresponding apparatus in the adjacent operators position circuit.

If the calling plugis` to be used' in establishing a connection; thebusy test will be made by rst touching the .tip of plug Itx tothe sleeveof the jack. If the jack circuit isalready busy, negative 'battery willbe impressed; through the tip of plug. I'I, break vcontact of ringingvkey 35, contact 8d, contact I'Voandithrough.either contact |99' or 20gto light the busy lampinthe same manner as described above.

Calling open loop subscriber Zine Before connecting theV calling cord toa subscribers line the operatorwill operate either key 24 or 25 toconnect a position circuit to the'cord as described above. When thecalling plug i! is connected to a subscribers line, ground will beconnected from the associated circuit to the sleeve of plug I I` and thecircuit extends through the Winding of relay I'Itobat-tery inparallelwith resistance 63. Relay II;Y operates. The operation of relayI'I establishes a circuit frombattery through contact IIa, resistance 62and the winding. of relay 9 to ground, operating relay 3. TheIoperation'of relay I'I also closes'the'transmission circuit of thecallingY end of the cord at contact IId. The circuit maybe tracedf fromthe tip of plug I I through the break contact of ringingV key 35,contact 8d, contact I'Id, contact Ita, resistance 43, bottom windingv ofpolar relay 23, armature of polar relay 22, contact 22m and resistance50 to positive battery. A cir-cuit is also established from ground onthering of thev line circuit through the ring of plug I I- and thewinding of relay It` to battery operatingY relay It'. Relay I5fdoes'notoperate at thistime, as direct ground' is ,connected from' the` positioncircuit through contact 24j or contact 25a tothe bottom terminal ofrelay I5. Therefore, battery supplied throughV contacts IIa andresistance 4IY finds a low resistance path shunting the path through thewinding of relay I5, contact Gc and contact IIic to ground. Theoperation of relay l5' pro- Vides a circuit extending from groundthrough contacts IGcZ andthe ilament of lamp 3i to battery, lightinglamp 3I-, which; remains lighted until the call is answered. Theoperationo relay I6' also establishes aI circuit from ground throughcontact la, contact I'Gb and contact 'Id to the conductor extending intothe night alarm circuit. The operation of relay I6 alsoV cnnectsnegative battery tothe transmissionA circuitk to hold polar repeaterrelay 23 on its marking contact pending the subscribers answer. Thecircuit may be traced from negative battery-through resistance 49,contact2id, resistance 42, contact I 6a', bottom winding of polarrepeater relay 23', armature or relay 22, contact 22m and resistancey 58to positive battery.

The subscriber is signaled by actuating ringing key 35 to close itsright-hand contact. This establishes a circuit from a. common ringingsupplyf for all of the cords, which extends from ground through thealternating current ringing generator' 36, lament of lamp 31, right-handcontact and armature of ringing key and the tip of calling plug I I. Thetip circuit of the subscribers line circuit to which calling plug II isconnected is grounded at the subscribers station throughV a condenser inseries with a ringer, which ringer is actuated by the alternatingringing current to attract the subscribers attention. When ringing key35 was operated to ring the subscribers station, a circuit was alsoestablished to operate relay 6. This circuit extends from batterythrough resistance 49, contactvZId, left-hand winding of relay 5,left-hand make contact of key 35, contact 8d, contact IId, contact I8a,resistance 43, bottom winding of polar relay 23, armature of polar relay22y contact 22m and resistance to .positive battery. In parallel with aportion of this circuit, between the bottom terminal of the left-handwinding of relay v 6 and the bottom terminal of resistance 43, is asecond branch extending through resistance 42 and contact I6a. Whenrelay 6 operates it locks from battery through its right-hand winding,contact 6b and contact I6c to ground. The operation of relay 6 alsoopens the path through the winding of relay I5. When the subscriberresponds, the transmission loop circuit extending through thesubscribers station is closed. This actuates the relays in thesubscribers line circuit at the central station to disconnect groundfrom the ring circuit to which the ring of plug II is connected. This,in turn, releases relay I6.

The release of relay I6, when the subscriber answers, opens contact I6athrough which negal tive battery was supplied to the right-hand terminalof the bottom winding of Vpolar relay 23, to hold the armature of thisrelay 'on its marking contact during the interval pending the answeringof the call by the subscriber. VWhen the subscriber answers, the tip andring conductors of the line circuit at the subscribers station areconnected. The ring circuit ofthe subscriber loop is terminated innegative battery. This negative battery takes the place of the negativebattery supplied through resistance 49 to holdthe armature of polarrelay 23 on marking and the armature is thereafterV held in engagementwith its marking contact by the negative battery supplied around. thesubscribers loop circuit to the tip of the calling cord.

The release of relay I6, when the subscriber answers, releases relay 6,by opening contact I6c which supplies ground to the holding winding Y ofrelay 6. The release of relay I6 also breaks the circuit heretoforetraced through the iilament of lamp 3|, extinguishing the lamp, as .asignal that the subscriber has answered. The release of relay I6 alsoopens the circuit to the night alarm heretofore traced. The transmissionpath through the calling side of the repeater is now established.

When it is necessary to ring on the answering cord, the position circuitis connected to the cord by operating key 24 or 25. A key is operated inthe position circuit to control a relay in the position circuit whichconnects continuous ringing current from the position circuit to the tipor transmitting conductor of the answering side of the cord.

Answering incoming call from an automatic signaling intertoll trunk Nowthe manner in which the cord circuit herein functions when it is used'toanswer a @all incoming over an automatic signaling trunk will bedescribed. When the answering plug I of the cord herein is connected toan automatic signaling intertoll trunk, all three conductors, tip, ringand sleeve, of the plug engage corresponding conductors connected to thejack of the trunk. The tip and sleeve connections in the trunk are thesame as described above. The ring connection in the trunk extends toground through a high resistance. Relays 4 and 5 operate in the samemanner as described above. Relay 2 operates over a circuit from the highresistance ground on the ring of the jack through the windings of relays2 and I2 to battery. Relay I2 does not operate due to the highresistance in the ring circuit. The operation of relay 2 closes acircuit which extends from ground through contact I2a, contact 2a andthe winding of relay I to contact I'Ia which is open. The reason forthis will become apparent below.

The operation is otherwise the same as described above for theconnection to an open loop subscribers line.

Calling open Zoop subscriber when the answering cord is Vconnected toautomatic signaling intertoll trunk The cord circuit herein may be usedto complete a call incoming over an automatic signaling intertoll trunkto an open loop subscribers line. Under such circumstances a signalassociated with the incoming end of an automatic signaling intertolltrunk will indicate that a call is incoming over such a trunk. Theoperator will respond by connecting plug I of the answering cord to thejack of the incoming automatic trunk. For this condition as describedabove, all three conductors, tip, ring and sleeve of the answering plugI will be engaged. The ring and sleeve will each be connected to groundin the automatic trunk. The ring of the automatic trunk is grounded forthis condition through a high resistance. Relay 2 will operate from thisground over a circuit through the windings of relays 2 and I'2 tobattery. Relay I2 will not operate due tothe high resistance in the ringof the connected circuit. Relay 5 and relay 4 will both be operated byground connected to the sleeve of the plug over a circuit heretoforetraced. Relay 5 will lock as described heretofore. When the call fromthe automatic trunk, to which the answering plug I is connected, isextended by means of plug II to the subscribers line all threeconductors, tip, ring and sleeve will be engaged. Relay I'I will beoperated from ground connected to the sleeve of the subscribers line.The operation of relay I'I closes the transmission circuit at contactIId. The operation of relay I'I also operates relay 9. These circuitshave been traced heretofore. Since relay 2 was operated when plug I wasconnected to the automatic trunk, the operation of relay I"I alsooperates relay 'I over a circuit which may be traced from batterythrough contacts I 1a, winding of relay 1, contact 2a and contact I2a toground.

Relay I6 is operated from ground on the ring conductor of the linecircuit through the ring of plug II and the winding of relay I6 tobattery. As either key 24 or key 25 was operated before the plug wasinserted in the subscribers line jack, ground is supplied from theposition circuit through either Contact 24j or 25a to the bottomterminal of relay I 5. The closing of contact Ila, therefore, whichestablishes a circuit from battery through contact I'Ia and resistance4I to the bottom terminal of relay I5, does not operate relay I5, as thedirect ground supplied from the position circuit shunts the path toground through the winding of relay l5, contact 6c and Contact |Ec. Theoperation of relay I6 lights lamp 3| over a circuit from battery throughthe lament of the lamp and contact I'd to ground. Lamp 3| will remainlighted until the subscriber answers, when it will be extinguished in amanner to be described below. The operation of relay i6 also suppliesnegative battery to hold the armature of relay 23 in engagement with itsmarking contact during the interval before the calling subscriberanswers. The negative battery is supplied over a circuit extendingthrough resistance 49, contact Qld, resistance 42 and contact Ia to theright-hand terminal of the bottorn or line winding of polar relay 23.The operation of relay |6 establishes a circuit which may be traced frombattery through the winding of relay 8, contact 1c, contact |6b, contact|9c and contact 2|lc to ground operating relay 8. The operation of relay8 opens the sleeve circuit of the answering cord at contact 8b totransmit a supervisory signal back over the circuit connected to plug tothe originating cord. The operation of relay 8 also disconnected thepath to ground through the sleeve of the automatic trunk circuit andsubstituted instead ground through contact 8a, to hold relays 4 and 5operated. The operation of relay 8 also supplies ground to the nightalarm circuit through contact 8c. The subscriber is signaled in the samemanner as described above for calling an open loop subscribers line.

When the subscriber answers, ground is disconnected from the ringcircuit of the subscribers line which opens the path through the windingof relay I6, releasing relay I6. The release of relay I6 extinguishescalling lamp 3|' to indicate that the called subscriber has responded.The release of relay also opens the path from negative battery throughcontact Ilia which was holding the armature of polar relay 23 on itsmarking contact during the interval before the subscriber responded. Therelease of relay |6 also releases relay which Was locked through contact|6c. lThe release of relay I6 also releases relay 8, the operating pathfor which extended through contact lh. The release of relay 8 restoresthe sleeve circuit of the answering cord to normal and again transmits asupervisory signal to the originating ofiice to indicate that thesubscriber has answered. The

release of relay 8 also removes the ground from the night alarm circuit.The cord is now ready for communication.

Ringing the subscriber from the outward operator When answering cord isconnected to an automatic signaling intertoll trunk and calling cord isconnected to a subscribers line and a disconnect signal exists from thesubscriber, the outward operator at the distant end of the intertolltrunk can signal the subscriber by transmitting 20-cyc1e alternatingcurrent ringing from her cord circuit. This ringing current will betransmitted over the automatic signaling intertoll trunk to the localcord circuit.

At any time when it would be necessary to ring the subscribers line towhich the calling cord is connected, the line would be in the disconnectcondition. By this is meant that the tip circuit of the line would beconnected to ground through a condenser and a ringer at the subscribersstation. The transmission path through the subscrbers station would beopen. The ring and sleeve conductors, to which the ring and sleeve ofthe calling plug would be connected, would each be terminated in ground.Relay 8 would be operated in the manner described above. This wouldtransmit a supervisory signal back to the originating cord as 'a signalthat the connection was established and that the originating operatorshould ring. Twenty-cycle reversals of direct current incoming from theautomatic signaling intertoll trunk will aetuate the armature of relay22 between its marking and spacing contacts at 20-cycles per second.This will send pulses of positive and negative current at 20- cycles persecond over a circuit which extends `from the marking and spacingcontacts of polar relay 22 through the armature 4of relay 22, bottomwinding of relay 23, contact Hd, contact 8f, winding of relay iii andcondenser S5 to ground operating relay I0. The operation of relay I!)will connect a direct ground to the top terminal of relay 9, therebyshunting the path through the winding of relay` 9, which was operated ina manner described above, releasing relay 9. A circuit will then beestablished from ground through a source of alternating ringing current36, lament of lamp 31, contact 9a, contact 8e, break contact andarmature of ringing key 35 and the tip of calling plug out over the tipof the subscribers line to actuate the ringer in which the tip of thesubscribers line is terminated. Relay I6, which is operated from groundon the connected ring circuit, supplies negative battery to theright-hand terminal of polar relay 23, in the manner described above, tohold the `armature of relay 23 in engagement with its marking contactuntil the subscriber answers.

During the periods between the ringing intervals, relay H) will release.This disconnects the direct ground from the top terminal of the windingof relay 9 `and relay 9 will reoperate. While relay 9 is reope'ratedpositive battery will be supplied over a path through resistance 50,contact 9b and contact 8c to the tip of the calling cord. When thesubscriber answers ground will be disconnected from the ring and sleeveconductors of the subscribers line releasing these relays. This, inturn, releases relays 8 and 9 as described above and the circuit isconditioned for communication.

Calling on ringdown intertoll trunk Alow resistance relay winding in theringdown trunk. Relay operates but relay I6 remains unoperated. Incalling on such a trunk ringing source 36 is not employed. The callsignal is transmitted by opening the tip or transmitting conductor foran interval of two seconds. This is performed by first operating eitherkey 24 or key 25 to connect an operators position circuit in series withthe transmission path through the answering end of the cord. A key inthe operators position circuit is operated to open the transmission loopthrough the position circuit under control of a relay therein. When thetransmission path through the bottom Winding of polar relay 22 is thusopen the armature of relay 22 is actuated so as to engage with itsspacing contact,

Aof relay 23.

under the influence of current through its top or biasing winding. Thiscondition is maintained for two seconds resulting in the reception of acall signal at the distant end of the ringdovvn trunk.

Transmission through cord circuit Teletypewriter communication throughthe cord circuit is effected by operation of the single line polarrepeater relays 22 and 23, which are arranged with local circuits whichserve to bias the relay when the associated line circuit is closed andto hold the relay when the associated line circuit is open. Thus intransmitting from the answering end of the cord toward the calling endof the cord, when the armature of relay 22 is on its right-hand ormarking contact a biasing potential from positive battery throughresistance l), armature of relay 22, top winding of relay 23, resistance45 and contact 2lb is supplied to the top winding of relay 23. Thisopposes the effect of the line current through the bottom winding Theeffect of the line current is greater than the effect of the biasingcurrent and the armature of relay 23 is held in engagement with itsleft-hand or marking contact. When the armature of relay 22 is actuatedso as to engage with its left-hand or spacing contact the current in thebottom or line winding of relay 23 is reduced to zero. But the polarityof the current supplied from negative battery through resistance 52,contact 22s, armature of relay 22, top winding of relay 23, resistance45, and contact 2lb is reversed and its effect holds the Varmature ofrelay 23 in engagement with its left-hand or marking contact.

In the transmitting from the calling end of the cord toward theanswering end of the cord, when the armature of relay 23 is inengagement with its left-hand or marking contact, positive battery issupplied through resistance 5I, contact 23m.,v

armature of relay 23, top winding of relay 22, resistance 44 and contact2 Ial to ground. The effect of this ycurrent opposes theline current inthe bottom winding of relay 22, but the effect of the line currentpreponderates and the armature of relay 22 is held in engagement withits right-hand orv marking Contact. When the armature of relay 23 isactuated to engage with its right-hand vor spacing contact negativebattery is supplied through resistance 53, contact 23s, armature ofrelay 23, top winding of relay 22, resistance 44 and contact 2Ia toground. The line current through the bottom winding of relay 22 has beenreduced to zero, but the effect of the current through the top windingof relay 22, since its direction has been reversed, holds the armatureof relay 22 in engagement with its right-hand or marking contact.

Recall signal The cord circuit Vherein is arranged to receive a recallsignal over either the answering end of the cord or the calling end ofthe cord. The condition requisite for the reception of a recall signalin each case is that direct ground be connected momentarily to the ringof the associated answering plug I or calling plug II from the connectedcircuit, followed by the restoration of an open or high resistanceground condition. Attention is called to the fact that in the case ofany connecting circuit in which the ring circuit may be open for anincoming call, the connecting circuit must be arranged to connect groundto the ring of the cord to operate the supervisory signals in the cord.

In the case of a recall signal received over the answeringlend of thecord the direct ground on the ring conductor will operate relays 2 andI2. If received over the calling end of the cord the direct ground onthe ring conductor will operate relay I6. If the momentary ground occurson the ring of calling cord II and answering cord I is connected to anautomatic signaling intertoll trunk, the operation of relay I6 Will befollowed by the operation of relay 8 in a manner described above.Theoperation of relay 8 in turn will open the sleeve circuit of theanswering cord at contact 8b to transmit a supervisory signal back tothe originating cord. The operation of relay I6 will in turn operaterelay I5 over a circuit from negative battery through contact I'Ia,resistance 4 I, winding of relay I5, contact 6c and contact I6c toground. When relay I5 operates it locks to ground through contact I5b.Upon the removal of the direct ground from the ring of the plug ii,relay I 6 releases and establishes a path for flashing lamp 3 I as arecall signal. This path may be traced from battery through the filamentof lamp 3|, contact I6e and contact I5a to ground through interrupter10.

If the recall signal is received from the circuit to which plug I :isattached the momentary ground operates relays 2 and I2 which in turnoperates relay I4. The circuit may be traced from negative batterythrough contact I3a, resistance 40, winding of relay I4 and contact I2bto ground. Relay I4 locks through contact I4b. The removal of thetemporary direct ground from the ring of plug I releases relays2 and I2.This establishes a circuit for the flashing of lamp 30 as a recallsignal. The circuit may be traced from ybattery through the lament oflamp 30, contact I 2j and contact I4a to ground through interrupter 1U.

When lamp 3U or lamp 3I is flashed to recall the operator the ilashingof the lamp may be terminated by operating the home typing key 24 or theposition typing key 25. This, as has been shown, results in theconnection of direct ground from the position circuit to the bottomterminal of relay I5, shunting the path through the winding of relay I5,as described above, and releasing the relay. It also results in theconnection of direct .ground to the bottom terminal of relay I4 toperform the same function in the same manner. When relay I4 0r relay I5is released, the path to ground through interrupter 'I0 is opened forthe associated lamp and the iiashing is terminated.

The cord circuit herein is arranged for transmitting a recall signalover a connected ringdown intertoll trunk connection. In performing thisoperation typing key 24 or 25 is operated to connect the associatedposition circuit to the cord circuit. Then the transmission circuitthrough the associated position circuit is opened for a measuredinterval by controlled means in the associated position circuit. Thisresults in the transmission of a spacing signal from the repeater in thecord circuit for an interval of seven seconds as a recall signal overthe ringdown intertoll trunk.

Receiving a disconnect signal A disconnect signal is received as asteady direct ground over the ring of the plug connected to the circuittransmitting the disconnect signal. The steady direct ground willoperate either relays 2 and I2 or relay I6 depending upon whether thedisconnect signal is transmitted by the circuit to Which the plug I isconnected or by the circuit to which plug II is connected. The

4nect signal.

operation of relay I2l lights lamp 30 as a discon- The operation ofrelay I6 lights lamp 3| as a disconnect signal. I he circuits haveheretofore been traced. Since neitherkey 24 nor key 25 is operated atthis time ground is not provided for shunting the path through thewinding of relay i4 or relay I 5 as described above and these relaysoperate at this time. However, since relay I2 and relay I6 remainoperated due to the steady ground connected to the corresponding ringcircuit for this condition, the circuit for each lamp remains terminatedin steady ground supplied through the contact of relay I 2 or I6 tomaintain the lamp continuously lighted and the operation of the hashingrelay I4 or I5 performs no function at this time.

In the description above it has been pointed out that a steady lamp isused as a disconnect signal and a flashing lamp as a recall signal.There is of course no reason why a flashing lamp may not be used as adisconnect signal and a steady lamp as a recall signal if thecooperating circuits are arranged to supply the proper conditions to thering and sleeve circuits of the cord circuit herein to bring in adashing lamp as a disconnect signal and the steady lamp as a recallsignal.

As a matter of fact in cooperating with one particular type of circuit,namely, a carrier-operated toll line station circuit, the conditionsimposed on the ring circuit of the cord circuit herein are such thatlamp 3U or lamp 3| is flashed to indicate a disconnect signal. In orderto eifect this a momentaryr direct current is connected to the ring ofthe associated plug of the cord herein to operate either relay I 2 orrelay I6 and then the direct ground is removed to release relay I2 orrelay I6 to bring in a flashing lamp as described above. The onlydifference of course is in what the signals indicate. The

flashing lamp in this instance indicates a disconnect rather than arecall signal.

Through supervision The cord circuit herein as indicated above isarranged to pass a supervisory signal received from a circuit to whichone plug is connected directly through the cord to another circuit towhich the other plug is connected. For instance, when answering plug Iis connected to an automatic signaling intertoll trunk and the callingplug II is connected to another circuit, a disconnect signal receivedfrom the circuit to which calling plug II is connected will betransmitted through the cord circuit directly to the automatic signalingintertoll trunk. This is effected by the opening of the sleeve circuitof answering plug I at contact 8b when relay 8 is operated. The mannerin which relay 8 is operated is described above for such a connection.

Transmitting a disconnect signal for rinydown trunks A disconnect signalfor ringdown intertoll trunks is transmitted by momentarily operatingkey 34. This establishes a circuit from ground through the contact ofkey 34 and the right-hand winding of relay I8 to a conductor common forall cords which extends through the made contacts of a relay in theconnected position circuit to battery operating relay I8. When relay I8is operated it locks over a circuit from battery through its right-handWinding and contact I8c, through a conductor, also common to all cords,which extends into the position circuit through the; winding ei a relaytherein t0 gefilmd, through the-made contacts of al second relay in theposition circuit. The vrelaysv in the position circuit control aselector. 'I'he selector is operated for a period of ten seconds. Duringinterval relay vI8 remains operated opening the tip or transmissioncircuit of the calling end of the cord. The operation of relay I8 alsoestablishes a circuit from negative battery through resistance 4S,contact ZId, resistance 42, contact Ib and resistance 43 to theright-hand terminal of the bottom winding of polar relay 23 to hold thearmature of relay 2 3 in engagement with its marking Contact during theten second interval. The ten second open interval of the tip circuitserves as a disconnect signal which is transmitted over the circuit towhich calling plug II is connected.

Conference connection The cord circuit herein is arranged forestablishing a conference connection by means of which it is possiblefor a group of subscribers, such as three or more, to communicatesimultaneously. The conference circuit itself is Wellknown in the art.In order to establish a conference connection a calling Darty isanswered by means of answering plug I. After it has been establishedthat the calling party wishes to communicate with a group of subscriberssimultaneously, calling plug II is connected to one of the means ofaccess to the well-known conference circuit. Other subscribers areinterconnected to other means of access t0 the same conference circuit.Since the conference circuit is wellknown in the art, it is not thoughtnecessary to describe this operation in detail.

Description of functions of various other apparams the winding of relayI'I is a non-inductive resistance shunt around the winding of relay I'Iin the sleeve circuit to prevent shocks when connections are beingestablished by means of plug II.

Resistance 66 and condenser 61 are connected to the sleeve of plug I toreduce the possibility of shock while connections are being establishedthrough plug I. Condenser 54 and resistance 55 shunt contacts 22s toprotect these contacts against the effect of sparking. Condenser 5'I andresistance 56 shunt contacts 22m, condenser .58 and resistance 53protect contacts 23s and condenser 6I and resistance 60 protectscontacts 23m for the same reason.

The function of condensers 68 and 69 which shunt resistances 44 and `45in the artificial line circuits of polar relays 22 and 23 is to provideanti-kickoi protection.

What is claimed is:

l. In a manual teletypewriter switching system, a manual teletypewritercord circuit, a calling plug and an answering plug both in said circuit,means in said circuit for registering in said circuit a supervisorysignal impressed on said calling cord and means also in said circuit forsimultaneously transmitting said supervisory signal through saidanswering cord by way of a path completely independent of the telegraphsignal transmission path in said cord.

2. In a manual teletypewriter switching system, a manual teletypewritercord circuit, a calling plug and an answering plug both in said circuit,means in said circuit for registering a supervisory signal receivedthrough one of said Resistance 63 which is connected in shunt with plugsand means also in said circuit vfor simultaneously transmitting saidsupervisory signal from one to the other of said plugs by way of a pathcompletely independent of the telegraph signal transmission path in saidcord.`

3. In a manual teletypewriter Vswitching system, a manual teletypewritercord circuit, means in said cord circuit for iiexibly interconnecting avcalling and a called circuit, means in said cord circuit forregistering a disconnect signal, means in said cord circuit forregistering a recallsignal different from said disconnect signal, andmeans in said cord circuit for transmitting said disconnect and recallsignals through said cord circuit by way of a path lcompletelyindependent of the telegraph signal transmission path in said cord toone of said connected circuits as.

said supervisory signals are registered in said cord circuit.

V4. In combination in a manual teletypewriter cord circuit, an answeringand a, calling plug, a telegraphy channel including a telegraph repeaterinterconnecting said plugs, a, ringing supply circuit, means responsiveto an electrical condition impressed on one of said plugs for connectingsaid ringing supply circuit to the other of said plugs, a supervisorychannel independent of said telegraph channel, interconnecting saidplugs, means responsive to an electrical condition imposed on one ofsaid plugs, for trans- -mittinga signal through said supervisory channelto the other of said plugs and means responsive to an electricalcondition imposed on one Vof said plugs for preventing the transmissionof a signal receivedjby the other of said plugs, through saidsupervisory channel.

5.v In a teletypewriter cord circuit, a telegraph channel, a supervisorychannel completely independent of said telegraph channel, means in saidcord, responsive to a rst condition' imposed on saidY cord uponconnection to a rst'circuit, for preconditioning saidsupervisory'channel to transmit supervisory signals completely throughsaid supervisory channel between calling and answering plugs in saidcord, when supervisory signals are impressed on saidjcord and means insaid cord, responsive to a second condition imposed on said cord, uponconnection to a second circuit, lfor preconditioning said supervisorychannel to prevent the transmission of supervisory signals completelythrough said supervisory channel bewteen calling and called plugs insaid cord when supervisory signals are impressed on Said cord. v Y

6. In a teletypewriter switching system, a cord circuit, an answeringand a calling end on said cord circuit, a supervisory signal associatedwith one of said ends, a-telegraph signal transmitting channelinterconnecting said ends, a supervisory f2',eo,4

channel, completely independent oi said tele- Vimpressing acorresponding condition on-the opposite end of said cord through saidsupervisory channel.

7. In a manual teletypewriter switching system, a cord circuit, a rstanda second flexible manual switching means in said cord for extending acall between a calling anda called subscriber, a telegraph signaltransmitting channel in said cord interconnecting said switching means,a telegraph repeater in said channel, a supervisory signal channel insaid cord, completely independent of said telegraph channel,interconnecting said switching means for transmitting supervisorysignals from one to the other of said switching means, and selectivelyoperated means responsive only to the connection of one of saidswitching means to a particular kind of circuit to which a supervisorysignal is to be transmitted for conditioning said supervisory channel totransmitl said signal.

8. In a manual teletypewriter switching system, in combination in amanual teletypewriter cord circuit, a switching means conditioned toanswer incoming calls, relay means directly connected to said switchingmeans for preventing the simultaneous operative connection of twointerconnected cords, selectively operated relay means also directlyconnected to said switching means responsive to the connection of saidswitching means to a particular kind of circuit only for conditioningsaid cord to transmit a supervisory signal when received from thecalling end of said oord, by way of a completely independent supervisorychannel, through said answering switch- Ving means. l. l

9. In a teletypewriter cord circuit, in combination, an answering'endequipped with a plug, a plurality of conductors attached to said plug,means in said cord for preventing telegraph transmission through saidcord when said plurality of conductors of two identical independentcords are interconnected directly through multiple jacks, flashingrecall means associated with said answering end, means in said cord forconnecting an alternating current ringing supply circuit to the callingend of said cord in response to an electrical condition impressed on oneoi said conductors and means for impressing a supervisory signalcondition on one of said conductors in response to a supervisorysignaling lcondition imposed on the calling end of said cord.

CARL A. DAHLBOM. FULLERTON S., KINKEAD. GEORGE A. LOCKE.

